A liquid crystal display device used for a liquid crystal television, a liquid crystal display, etc. usually has a liquid crystal alignment film to control the liquid crystal alignment state provided therein. As the liquid crystal alignment film, a polyimide liquid crystal alignment film obtained by applying a liquid crystal aligning agent containing as the main component a solution of a polyimide precursor such as a polyamic acid (polyamide acid) or a soluble polyimide to a glass substrate or the like and baking it is mainly used. At present, by the industrially most common method, a liquid crystal alignment film is prepared by carrying out so-called rubbing treatment such that the surface of a polyimide liquid crystal alignment film formed on an electrode substrate is rubbed in one direction with a cloth of e.g. cotton, nylon or polyester.
A method of applying a rubbing treatment to the film surface in the alignment procedure of the liquid crystal alignment film, is an industrially useful method which is easily conducted and is excellent in the productivity. However, demands for higher performance, high definition and increase in the size of a liquid crystal display device are increasing, and various problems have been revealed such as scars on the surface of the alignment film formed by the rubbing treatment, influences by dust, mechanical force and static electricity, and non-uniformity in the alignment film plane.
As an alternative to the rubbing treatment, a photo-alignment method of imparting liquid crystal alignment performance by irradiation with polarized ultraviolet rays has been known. As the liquid crystal alignment treatment by the photo-alignment method, in terms of mechanism, one utilizing photoisomerization reaction, one utilizing photocrosslinking reaction, one utilizing photodecomposition reaction, etc. have been proposed (Non-Patent Document 1). Further, in Patent Document 1, use of a polyimide film having an alicyclic structure such as a cyclobutane ring in its main chain for photo-alignment method has been proposed.
The above photo-alignment method is not only advantageous in that a liquid crystal alignment film can be produced by an industrially easy production process but also has the following advantages. That is, in an in-plane switching (IPS) mode liquid crystal display device in which an electric field is applied to a substrate in a horizontal direction (a transverse direction) for switching the orientation of liquid crystal molecules or a fringe field switching (hereinafter referred to as FFS) mode liquid crystal display device, by using a liquid crystal alignment film obtained by the above photo-alignment method, as compared with a liquid crystal alignment film obtained by the rubbing treatment method, improvement of the contrast and the viewing angle of a liquid crystal display device can be expected.
A liquid crystal alignment film to be used for an IPS mode or FFS mode liquid crystal display device is not only required to have basic properties such as excellent liquid crystal alignment property and electrical properties but also required to suppress a residual image which occurs due to long-term alternative-current drive in an IPS mode or FFS mode liquid crystal display device.
However, a liquid crystal alignment film obtained by the photo-alignment method is problematic in that the anisotropy to the alignment direction of a polymer film is small as compared with a film obtained by rubbing. If the anisotropy is small, no sufficient liquid crystal alignment property will be obtained, and when such a liquid crystal alignment film is used for a liquid crystal display device, problems may arise such that a residual image occurs. Thus, as a method for increasing the anisotropy of a liquid crystal alignment film obtained by the photo-alignment method, removal of low molecular weight components (decomposed products) formed by cleavage of the main chain of the polyimide by irradiation with light, by heating, has been proposed (Patent Document 2).
Further, in order to broaden the effective pixel area of a liquid crystal display device in recent years, it has been required to reduce a so-called picture frame area on which pixels are not formed on the peripheral edge portion of a substrate. As the picture frame of a panel is narrowed, a sealing agent for bonding two substrates to prepare a liquid crystal display device is applied on a polyimide liquid crystal alignment film, however, since a polyimide has no polar group, no covalent bond is formed between the sealing agent and the surface of the liquid crystal alignment film, and adhesion of the substrates may be insufficient.